Change proc_exit to unwind the stack rather than exiting the host process. (#1646)

* Remove Cranelift's OutOfBounds trap, which is no longer used.

* Change proc_exit to unwind instead of exit the host process.

This implements the semantics in https://github.com/WebAssembly/WASI/pull/235.

Fixes #783.
Fixes #993.

* Fix exit-status tests on Windows.

* Revert the wiggle changes and re-introduce the wasi-common implementations.

* Move `wasi_proc_exit` into the wasmtime-wasi crate.

* Revert the spec_testsuite change.

* Remove the old proc_exit implementations.

* Make `TrapReason` an implementation detail.

* Allow exit status 2 on Windows too.

* Fix a documentation link.

* Really fix a documentation link.

crates/wasi-common/src/old/snapshot_0/hostcalls_impl/misc.rs

@@ -342,13 +342,6 @@ pub(crate) struct FdEventData<'a> {
     pub(crate) userdata: wasi::__wasi_userdata_t,
 }
 
-pub(crate) fn proc_exit(_wasi_ctx: &WasiCtx, _memory: &mut [u8], rval: wasi::__wasi_exitcode_t) {
-    trace!("proc_exit(rval={:?})", rval);
-    // TODO: Rather than call std::process::exit here, we should trigger a
-    // stack unwind similar to a trap.
-    std::process::exit(rval as i32);
-}
-
 pub(crate) fn proc_raise(
     _wasi_ctx: &WasiCtx,
     _memory: &mut [u8],

crates/wasi-common/src/snapshots/wasi_snapshot_preview1.rs

@@ -813,13 +813,10 @@ impl<'a> WasiSnapshotPreview1 for WasiCtx {
         Ok(nevents)
     }
 
-    // This is just a temporary to ignore the warning which becomes a hard error
-    // in the CI. Once we figure out non-returns in `wiggle`, this should be gone.
-    #[allow(unreachable_code)]
-    fn proc_exit(&self, rval: types::Exitcode) -> std::result::Result<(), ()> {
-        // TODO: Rather than call std::process::exit here, we should trigger a
-        // stack unwind similar to a trap.
-        std::process::exit(rval as i32);
+    fn proc_exit(&self, _rval: types::Exitcode) -> std::result::Result<(), ()> {
+        // proc_exit is special in that it's expected to unwind the stack, which
+        // typically requires runtime-specific logic.
+        unimplemented!("runtimes are expected to override this implementation")
     }
 
     fn proc_raise(&self, _sig: types::Signal) -> Result<()> {

crates/wasi-common/wig/src/hostcalls.rs

@@ -17,6 +17,13 @@ pub fn define(args: TokenStream) -> TokenStream {
 
     for module in doc.modules() {
         for func in module.funcs() {
+            // `proc_exit` is special; it's essentially an unwinding primitive,
+            // so we implement it in the runtime rather than use the implementation
+            // in wasi-common.
+            if func.name.as_str() == "proc_exit" {
+                continue;
+            }
+
             ret.extend(generate_wrappers(&func, old));
         }
     }

crates/wasi-common/wig/src/wasi.rs

@@ -46,6 +46,15 @@ pub fn define_struct(args: TokenStream) -> TokenStream {
             linker_add.push(quote! {
                 linker.define(#module_name, #name, self.#name_ident.clone())?;
             });
+            // `proc_exit` is special; it's essentially an unwinding primitive,
+            // so we implement it in the runtime rather than use the implementation
+            // in wasi-common.
+            if name == "proc_exit" {
+                ctor_externs.push(quote! {
+                    let #name_ident = wasmtime::Func::wrap(store, crate::wasi_proc_exit);
+                });
+                continue;
+            }
 
             let mut shim_arg_decls = Vec::new();
             let mut params = Vec::new();
@@ -291,6 +300,15 @@ pub fn define_struct_for_wiggle(args: TokenStream) -> TokenStream {
             linker_add.push(quote! {
                 linker.define(#module_name, #name, self.#name_ident.clone())?;
             });
+            // `proc_exit` is special; it's essentially an unwinding primitive,
+            // so we implement it in the runtime rather than use the implementation
+            // in wasi-common.
+            if name == "proc_exit" {
+                ctor_externs.push(quote! {
+                    let #name_ident = wasmtime::Func::wrap(store, crate::wasi_proc_exit);
+                });
+                continue;
+            }
 
             let mut shim_arg_decls = Vec::new();
             let mut params = Vec::new();